C Preprocessor Questions and Answers

Q: What is the output of this program? #include <stdio.h> #define clrscr() 17 int main() { clrscr(); printf("%d",clrscr()); return 0; }

Correct answer: 17clrscr() is a function-like macro expanding to the constant 17. The first call becomes the statement 17;, which computes a value and discards it - legal, if pointless. The printf then expands to printf("%d", 17) and prints 17.

Q: If the file to be included doesn't exist, the preprocessor flashes an error message.

Correct answer: TrueIf the named file cannot be found on any of the search paths, the preprocessor reports a fatal error and translation stops - the familiar “No such file or directory”. Nothing reaches the compiler, so the missing header is discovered before any C is parsed.

Q: A preprocessor directive is a message from programmer to the preprocessor.

Correct answer: TrueA directive such as #include or #define is an instruction to the preprocessor - the phase that rewrites the source text before the compiler sees it. It is not part of the C language proper, which is why it has its own syntax and ends at the end of a line rather than at a semicolon.

Q: A line in a program consisting of nothing but a # represents

Correct answer: a null directiveA # on a line by itself is the null directive: the preprocessor recognises it, does nothing, and removes it. It is legal and occasionally used as a spacer inside a run of conditionals to keep the nesting readable.

Q: Identify the trigraph sequence for ^.

Correct answer: ??'The trigraph for the caret is ??'. The full set is ??= for #, ??/ for backslash, ??' for ^, ??( and ??) for the square brackets, ??! for |, ??< and ??> for the braces, and ??- for ~.

Q: The preprocessor can trap simple errors like missing declarations, nested comments or mismatch of braces.

Correct answer: FalseThe preprocessor does not parse C at all. It handles directives, expands macros and strips comments; it has no notion of declarations or of matching braces, so those errors surface later, in the compiler. It does report its own faults, such as a missing include file or an unclosed #if.

Q: The C-preprocessors are specified with _________symbol.

Correct answer: #Every preprocessor directive begins with # as the first non-blank character of the line. The symbol is what marks the line as an instruction to the preprocessor rather than a piece of C.

Q: What is the output for the following code snippet? #include <stdio.h> #define A -B #define B -C #define C 5 int main() { printf("The value of A is %d\n", A); return 0; }

Correct answer: The value of A is 5Macros are rescanned after expansion, so the substitution cascades:A → -B → -(-C) → -(-5)Two negations cancel, so the value printed is 5.

Q: What will be the output of the program? #include <stdio.h> #define MAX(a, b, c) (a>b ? a>c ? a : c: b>c ? b : c) int main() { int x; x = MAX(3+2, 2+7, 3+7); printf("%d\n", x); return 0; }

Correct answer: 10The macro expands to a nested conditional. With $a = 3+2 = 5$, $b = 2+7 = 9$ and $c = 3+7 = 10$:(5>9 ? … : 9>10 ? 9 : 10)The first test fails, so the else branch is taken; $9 > 10$ also fails, leaving 10. Addition binds tighter than the comparisons, so the unparenthesised arguments happen to be safe here.

81.

What is the output of this program?

#include <stdio.h>
#define clrscr() 17
int main()
{
  clrscr();
  printf("%d",clrscr());
  return 0;
}

A.Compilation error
B.Runtime error
C.17
D.none of the above

Answer: C

Correct answer: 17

clrscr() is a function-like macro expanding to the constant 17. The first call becomes the statement 17;, which computes a value and discards it - legal, if pointless. The printf then expands to printf("%d", 17) and prints 17.

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82.

If the file to be included doesn't exist, the preprocessor flashes an error message.

A.True
B.False

Answer: A

Correct answer: True

If the named file cannot be found on any of the search paths, the preprocessor reports a fatal error and translation stops - the familiar “No such file or directory”. Nothing reaches the compiler, so the missing header is discovered before any C is parsed.

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83.

A preprocessor directive is a message from programmer to the preprocessor.

A.True
B.False

Answer: A

Correct answer: True

A directive such as #include or #define is an instruction to the preprocessor - the phase that rewrites the source text before the compiler sees it. It is not part of the C language proper, which is why it has its own syntax and ends at the end of a line rather than at a semicolon.

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84.

A line in a program consisting of nothing but a # represents

A.an invalid code
B.a null directive
C.a comment
D.a page number

Answer: B

Correct answer: a null directive

A # on a line by itself is the null directive: the preprocessor recognises it, does nothing, and removes it. It is legal and occasionally used as a spacer inside a run of conditionals to keep the nesting readable.

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85.

Identify the trigraph sequence for ^.

A.??/
B.??=
C.??'
D.??!

Answer: C

Correct answer: ??'

The trigraph for the caret is ??'. The full set is ??= for #, ??/ for backslash, ??' for ^, ??( and ??) for the square brackets, ??! for |, ??< and ??> for the braces, and ??- for ~.

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86.

The preprocessor can trap simple errors like missing declarations, nested comments or mismatch of braces.

A.True
B.False

Answer: B

Correct answer: False

The preprocessor does not parse C at all. It handles directives, expands macros and strips comments; it has no notion of declarations or of matching braces, so those errors surface later, in the compiler. It does report its own faults, such as a missing include file or an unclosed #if.

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87.

The C-preprocessors are specified with _________symbol.

A.#
B.$
C.” ”
D.&

Answer: A

Correct answer: #

Every preprocessor directive begins with # as the first non-blank character of the line. The symbol is what marks the line as an instruction to the preprocessor rather than a piece of C.

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88.

What is the output for the following code snippet?

#include <stdio.h>
#define A -B
#define B -C
#define C 5

int main()
{
  printf("The value of A is %d\n", A); 
  return 0;
}

A.The value of A is 4
B.The value of A is 5
C.Compilation Error
D.Runtime Error

Answer: B

Correct answer: The value of A is 5

Macros are rescanned after expansion, so the substitution cascades:

A → -B → -(-C) → -(-5)

Two negations cancel, so the value printed is 5.

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89.

Macro calls and function calls work exactly similarly.

A.True
B.False

Answer: B

False, A macro just replaces each occurrence with the code assigned to it. e.g. SQUARE(3) with ((3)*(3)) in the program.

A function is compiled once and can be called from anywhere that has visibility to the funciton.

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90.

What will be the output of the program?

#include <stdio.h>
#define MAX(a, b, c) (a>b ? a>c ? a : c: b>c ? b : c)

int main()
{
    int x;
    x = MAX(3+2, 2+7, 3+7);
    printf("%d\n", x);
    return 0;
}

A.3
B.5
C.10
D.3+7

Answer: C

Correct answer: 10

The macro expands to a nested conditional. With $a = 3+2 = 5$, $b = 2+7 = 9$ and $c = 3+7 = 10$:

(5>9 ? … : 9>10 ? 9 : 10)

The first test fails, so the else branch is taken; $9 > 10$ also fails, leaving 10. Addition binds tighter than the comparisons, so the unparenthesised arguments happen to be safe here.

Discuss in Forum

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